There are two types of optical recording media, one of which is to form physically deformed portions such as holes or concavities at a certain place of a recording layer by irradiation with beam of energy, and the other of which is to form portions having been changed in optical properties such as refractive index and reflectance at a certain place of a recording layer by irradiation with beam of energy.
Recording layers consisting essentially of low melting metal such as tellurium (Te) have heretofore been known as those used in either of the two types of the optical recording media (Japanese Patent L-O-P Publns. Nos. 71195/1983 and 9234/1983). Te coating, typical of low melting metallic coatings, is capable of forming thereon desired physically deformed portions or portions having been changed in optical properties (hereinafter generically called "bits") by using a very low energy, and thus is very useful as a high sensitivity material. By sensitivity as used herein is meant that which is defined by energy (mw/cm.sup.2) required for forming bits per unit surface area.
However, Te is oxidized with oxygen or moisture when allowed to stand in contact with the atmosphere, whereby it increases in transmittance and becomes transparent. Because of its thin coating as thin as about several hundreds .ANG., the recording layer formed by using Te alone markedly decreases in sensitivity when it increases in transmittance by oxidation of Te contained in such a thin recording layer. That is, when the recording layer composed of Te alone is oxidized, it increases in melt temperature and evaporation temperature and, at the same time, it becomes small in absorption of energy such as light as it becomes transparent, with the result that a large energy is required for forming bits and thus the recording layer markedly decreases in sensitivity. For instance, when Te coating formed on a substrate is allowed to stand in the circumstances of 70.degree. C. and 85% RH, its sensitivity decreases by about 20% in about 5 hours, and by about 50% in about 15 hours.
With the view of solving such problems as mentioned above, there are taken various measures to prevent oxidation of Te coating. It is known as one of these measures that Te coating is coated on the surface with a stable inorganic substance. Though this measure is effective in preventing Te coating from its being oxidized, it has not been put in practical use, because the use of the stable inorganic substance results in decrease of sensitivity of the Te coating and is also expensive. On one hand, it is also known to coat Te coating on the surface with plastics, but this measure is of little real use in preventing the Te coating from its being oxidized, because the plastics permit oxygen or moisture to permeate there through with relative ease, though the plastics are advantageous in that because of their low thermal conductivity, they are low in degree of marring the sensitivity of the Te coating.
With the view of solving such problems as referred to above, Japanese Patent L-O-P Publn. No. 63038/1984 discloses optical recording media having recording layers consisting essentially of Te and additionally containing Cr. As stated in this publication, it has been known that when Cr is contained in a recording layer consisting essentially of Te, oxidation resistance of the resulting recording layer is improved in proportion to the content of Cr in said recording layer, and thus duration of life of the optical recording medium having such recording layer as mentioned above may be prolonged.
Optical recording media having recording layers consisting essentially of Te and additionally containing Cr as disclosed in the above-cited publication, however, possess such a drawback that the recording sensitivity decreases when large amounts of Cr are contained in the recording layers. On that account, it has been a common practice from the standpoint of improvement in oxidation resistance and in recording sensitivity that the content of Cr to be contained in a recording layer consisting essentially of Te is decided so as to amount of 5-15% by weight (11-27 atom % based on the total atoms present in the recording layer) based on the Te present in the recording layer, as indicated in the above-cited Japanese Patent L-O-P Publn. No. 63038/1984.
However, the present inventors have found the fact that optical recording media having Te recording layers containing 5-15% by weight, based on the Te, of Cr are still low in recording sensitivity in comparison with an optical recording medium having a recording layer consisting only of Te. As a result of extensive researches conducted by the present inventors on such optical recording media as having recording layers consisting essentially of Te and additionally containing Cr, it has been found that optical recording media having recording layers consisting essentially of Te and additionally containing a specific amount of Cr have excellent recording sensitivity in comparion with those having Te recording layers containing large amounts of Cr and, at the same time, their recording margin sometimes enlarged.
Furthermore, the present inventors prosecuted extensive researches on optical recording media having recording layers containing Te and Cr (the content of Cr is not particularly limited), and eventually have found that a recording layer containing Te and Cr is formed on a substrate and then subjected to heat treatment, whereupon the recording margin is enlarged and the recording sensitivity is further improved. By "recording margin" as used herein is meant a breadth of the range of laser output used for forming bits of a predetermined shape. When this recording margin is broader, it becomes possible to form bits uniform in shape even when a laser output is varied, with the result that the range of the laser output usable at the time of information recording and consequently the resulting optical recording layer will exclude influences of irregularities of the laser output used and the like.